1. Field of the Invention
The present invention relates to a method for producing a catalyst containing alumina that can be used at high temperatures. In particular, it relates to a method for producing a catalyst that can be used for purifying exhaust gas to be discharged from internal combustion engines, etc.
2. Description of the Related Art
Heretofore, alumina (essentially .gamma.-alumina, .gamma.-Al.sub.2 O.sub.3) has been employed as a catalytic support to be used at high temperatures, since it has a large specific surface area and has good thermal stability. For instance, a catalyst for purifying exhaust gas to be discharged from internal combustion engines of cars, etc. is always exposed to high temperatures of about 850.degree. C., and even alumina only can be used as the support for the catalyst.
For fan heaters, gas turbine engines, etc., combustion techniques of using combustion catalyst have been developed so as to stabilize the combustion in them and to reduce nitrogen oxides (NOx) to be discharged from them. It is said that combustion catalyst for fan heaters require catalytic supports that can be used at 900.degree. to 1200.degree. C. while those for gas turbine engines require catalytic supports that can be used at 1100.degree. to 1500.degree. C. However, transition aluminas such as .gamma.-alumina, etc. that are popularly used as catalytic supports because of its large specific surface area, are converted into .alpha.-alumina (.alpha.-Al.sub.2 O.sub.3) at 1000.degree. C. or higher while being rapidly sintered to have a reduced specific surface area (see Catalyst, 29 [4] 293-298 (1987)).
Since simple oxides cannot satisfy the above-mentioned requirements, catalysts of composite oxides have been studied. For example, La-.beta.-Al.sub.2 O.sub.3 and BaO-6Al.sub.2 O.sub.3 are known to have relatively high heat resistance. La-.beta.-Al.sub.2 O.sub.3 having a ratio of La/Al of 5/95 has the highest thermal stability and has a specific surface area of 40 m.sup.2 /g at 1200.degree. C. However, it has a reduced specific surface area of 8 m.sup.2 /g at 1400.degree. C. BaO-6Al.sub.2 O.sub.3 has a specific surface area of 50 m.sup.2 /g at 1200.degree. C. but has a reduced specific surface area of 10 m.sup.2 /g at 1400.degree. C. (see Chemical Equipment, 29 [2] 134-137 (1987)).
Mullite composed of alumina and silica (3Al.sub.2 O.sub.3.2SiO.sub.2) is known as one of compounds that are the most stable at high temperatures, and it has been considered to use a porous material made of alumina and silica having the composition of mullite, as supports for catalysts, etc.
As the conventional method for producing a porous material made of alumina and silica having the mullite composition, there is known a method of mixing an alumina hydrosol and a silica hydrosol at the mullite composition (3Al.sub.2 O.sub.3.2SiO.sub.2) followed by sintering the resulting mixture (see Japanese Patent Laid-Open No. 3-266985).
The porous material produced by said known method has a specific surface area of 47 m.sup.2 /g at 1200.degree. C. but has a reduced specific surface area of 10 m.sup.2 /g at 1300.degree. C. Thus, the reduction in the specific surface area of this porous material at high temperatures is noticeable.
The reduction in the specific surface area of conventional porous materials containing alumina at high temperatures is large.
Having studied this phenomenon, we, the present inventors have considered the reasons of this phenomenon as follows:
The conventional porous materials containing alumina that have heretofore been used as catalytic supports are produced by a wet process, and the alumina in these materials is .gamma.-alumina. The specific surface area of this .gamma.-alumina is suddenly reduced at high temperatures of 1000.degree. C. or higher. This is because the .gamma.-alumina is converted to an .alpha.-phase at such high temperatures by phase transition, resulting in rapid growth of the alumina particles.
The .gamma.-alumina produced by a wet process comprises fine primary particles having a particle size of several to several tens .mu.m or less and therefore has a large specific surface area of 100 m.sup.2 /g or more, but the particles exist as firmly agglomerated secondary particles having a .mu.m-order particle size. Therefore, when once .alpha.-alumina is formed in these particles, the .gamma.-phase is easily converted into an .alpha.-phase in almost all the agglomerated secondary particles and the .alpha.-phase transition is rapid. When silica or oxides of alkaline earth metals or rare earth metals are added to such large secondary particles of .gamma.-alumina, the heat resistance of said .gamma.-alumina can be improved in some degree. However, since the .alpha.-phase transition occurs from the agglomerated sites and expands into the whole of the particles, the specific surface area of the resulting .alpha.-phase particles is reduced.
The same shall apply to .delta.-phase alumina and .theta.-phase alumina comprising a number of agglomerated particles.